The present invention relates generally to optical components employing optical fibers, and more particularly to optical collimators.
As the capacity required of wavelength division multiplexed (WDM) optical transmission systems continues to grow, the fiber optic component industry is confronted with increasingly stringent requirements for high performance fiber optic devices. In most cases one or more fibers are led into an optical device by an optical collimator, which provides low-loss transmission between the fiber and the optical device. In general and somewhat abstract terms, the input light entering the optical device via the collimator interacts with some optical component and the resulting light exits the optical device via one or more fibers that are also incorporated in optical collimators. One example of such an optical device is a wavelength division multiplexer that is based on an optical component such as a dielectric interference filter, which transmits or reflects selected wavelengths. As a basic building block of fiber optic devices, the performance characteristics of the optical collimators strongly heavily determines the level of performance of the fiber optic devices.
A commercially available optical collimator includes as its most fundamental components the fiber, a ferrule that holds the exposed fiber end, a collimating lens, an outer sleeve for containing the assembly and a means to position the fiber tip at the focal point of the collimating lens. Typically, a GRIN-type lens or cylindrical glass rod with an aspherical front surface is used as the collimating lens. A GRIN lens is fabricated to have a radially varying index of refraction that is greater towards the center, with the result being that it produces a focusing effect similar to a convex lens. The ferrule, including its associated fiber, and the lens are typically inserted in and bonded to a cylindrical sleeve so that the GRIN lens collimates the light diverging from the smaller core of the optical fiber. The collimator can then be inserted into the optical system with alignment provided by the sleeve. The fixed displacement between the fiber and the GRIN lens in the collimator should provide an optically well-characterized beam, and result in minimum insertion loss between the fiber and the optical system. Such collimators are often commercially available and are fabricated independently of the optical system in which they are to be used. Collimator assemblies typically have the output beam aligned parallel to the cylindrical outer sleeve with up to +/xe2x88x921.0 degree of tilt.
Optical systems often must receive light from two or more fibers, and thus an array of two or more collimators must be provided. In many cases it is important to align the optical output beams from the array of collimators so that they are all parallel to one another to a high degree of accuracy. Since the introduction of optical devices such as optical cross-connects and optical switches, the proper alignment of the optical beams has become a more stringent requirement. This is because the performance of optical cross-connects and switches is directly related to the stability and parallelism of the optical beams exiting the collimator array. Currently there is a need for collimator arrays whose optical beams are parallel to within 1.0 milliradian or better.
Various techniques have been employed to precisely align collimator arrays. Most of these techniques can be broadly classified into one of two categories based on whether beam alignment is performed in a passive or active manner. In the first category (passive alignment), piece-parts that are manufactured to meet stringent tolerances are employed. Proper alignment is achieved by the precise orientation and registration of the accurately manufactured parts. The piece-parts are usually secured in their proper orientation with epoxy. No active alignment of the individual piece-parts is performed. Collimator arrays with beam parallelism consistently better than about 1 milliradian are difficult to manufacture in accordance with these techniques because the piece-parts generally cannot be manufactured to the required tolerances. Also, consistent registration of the piece-parts to within micron or sub-micron tolerances is difficult or not even possible to achieve.
In the second category of collimator array alignment techniques, alignment is accomplished in an active manner using off-the-shelf piece-parts that only meet customary tolerances that are standard in their respective manufacturing industries. The desired beam alignment accuracy is achieved by adjusting the orientation of the piece-parts of the various collimators in the array while observing the optical beams they produce. Once alignment is achieved, the parts are secured in place with epoxy. Similar to the first category of techniques, collimator arrays with beam parallelism consistently better than about 1 milliradian are difficult to manufacture in accordance with this second category of techniques. A primary reason for this limitation is that the epoxy used to secure the components in place shrinks during the curing process. Typically, the variation in the dimensions of the piece-parts causes gaps that allow adjustment between piece-parts to achieve the desired alignment accuracy. When alignment is completed these gaps are typically non-symmetrical and are generally wedge-shaped. Since heat-cure epoxies shrink during curing, piece-part misalignments will occur and thus errors in the alignment of the optical beam will arise during the epoxy cure because of the non-symmetrical gaps between piece-parts. Even so-called xe2x80x9clow-shrinkxe2x80x9d epoxies shrink in the range of about 1-2%. Another problem is that the coefficient of thermal expansion of most epoxies that are used in practice is much greater than the coefficient of thermal expansion of steel or glass, which leads to alignment errors as the ambient temperature fluctuations.
Accordingly, there is a need for a collimator array that is assembled in a manner that minimizes or eliminates the clearances between piece-parts and/or avoids thick epoxy joints and wedges.
The present invention provides a method for assembling an optical collimator array. The method begins by directing light through a first optical collimator, which is supported by a first carrier element, to produce a first optical output beam. The first collimator is rotated about its central longitudinal axis to define a first angle between the first optical output beam and the central longitudinal axis. The first optical collimator is secured to the first carrier element and the first carrier element is secured to prevent rotation about the carrier axis. The method continues by rotating a second optical collimator that is supported by a second carrier element and which produces a second optical output beam. The second carrier element is rotated about a carrier axis perpendicular to the central longitudinal axis and in a plane containing the central longitudinal axis to define a second angle between the second optical output beam and the central longitudinal axis. The second optical collimator and the second carrier element continue to be rotated until a difference between the first angle and the second angle is less than a prescribed angular differential. Finally, the second optical collimator is secured to the second carrier element and the second carrier element is secured to the first carrier element.
In accordance with another aspect of the invention, a method is provided for assembling an optical collimator array. The method begins by directing light through a first optical collimator to produce a first optical output beam. The first collimator is supported by a first carrier element. The first collimator is rotated about its central longitudinal axis to adjust a position of the first optical output beam on a surface that intercepts the first optical output beam. The first carrier element is then rotated about a carrier axis perpendicular to the central longitudinal axis and in a plane containing the central longitudinal axis to further adjust the position of the first optical output beam on the surface. The first collimator continues to be rotated about these axes until the first optical output beam is located at a desired position on the surface, at which point the first optical collimator is secured to the first carrier element. Next, the first carrier element itself is secured to prevent rotation about the carrier axis. The aforementioned steps are repeated for a second optical collimator producing a second optical output beam, which is supported by a second carrier element. The second optical collimator continues to be rotated about the two axes until the second optical output beam is located at a second position on the surface that is offset from the position of the first optical output beam by a prescribed amount. Finally, the second optical collimator is secured to the second carrier element and the second carrier element is secured to the first carrier element.
In accordance with one aspect of the invention, the offset between the positions of the first and second optical output beams on the surface is selected so that the beams are parallel to within less than about 1 milliradian.
In accordance with another aspect of the invention, the offset between the positions of the first and second optical output beams on the surface is selected so that the beams are parallel to within less than about 0.3 milliradian.
In accordance with yet another aspect of the invention, the offset between the positions of the first and second optical output beams on the surface is selected so that the beams are parallel to within about 0.1 to 1 milliradians.